Gellan Gum is a microbial extracellular polysaccharide, produced by a Sphingomonas strain (Sphingomonas paucimobilis) via aerobic fermentation. This product was successfully developed by CP Kelco U.S., Inc. in 1978. It is another microbial extracellular polysaccharide after xanthan, which is non-toxic and safe, and shows good physicochemical properties. It was approved to be used in food products in Japan, as early as in 1988. The FDA of the USA government approved its use in foodstuff in 1992. In our country, it was approved to be used as a thickener or a stabilizer for food products in 1996. And there are more than ten of other countries, which have approved its use as an additive for food products. In recent years, gellan gum is increasingly and broadly used in food industry, medicine industry, chemical industry and other industrial fields, as a new emulsifier, a suspending agent, a thickener, a stabilizer, a gelling agent, a slow-released agent, a film-forming material and etc., due to its unique good properties. This shows a huge prospect of its applications being broadened.
Gellan gum is comprised of β-1,3-D-glucose, β-1,4-D-glucuronic acid and α-1,4-L-rhamnose in the mole ratio of 2:1:1, with the linking order of a β-1,3-D-glucose, a β-1,4-D-glucuronic acid, a β-1,3-D-glucose, and an α-1,4-L-rhamnose to form the tetrasaccharide unit. To the framework of long chains of sugars formed by the polymerization of tetrasaccharide units, there are acyl groups linked. The molecular weight of gellan gum normally is about 5×105-1×106 daltons. Gellan gum mainly exists in two forms, i.e., the non-deacylated high acyl gellan gum (also called as native gellan gum), and the physically and/or chemically artificially-deacylated low acyl gellan gum. In the high acyl or native gellan gum, there are two kinds of acyl groups, namely, acetyl and glyceroyl. Usually, the acetyl groups are linked to the C 6 position of the first glucose residue while the glyceroyl groups are linked to the C 2 position of the same glucose residue. In general, for each tetrasaccharide unit, there is one glyceroyl group on average, and 0.5 acetyl groups on average. The low acyl gellan gum is a product substantially free of acyl groups, produced from high acyl gellan gum by a deacylation treatment. So, the molecules of gellan gum have various molecule weights showing large differences, depending upon whether the molecules are deacylated or not and how much degree the molecules are deacylated. At present, the low acyl gellan gum with a relatively low molecular weight has the most applications in food industry.
The gellan gum-producing bacterial strains currently used in production, sphingomonas strains, can co-produce the by-products through metabolism during fermentation process, i.e., the yellow pigments (mainly the pigments carotinoids), which not only competes the carbon source with gellan gum, but only makes the fermentation broth yellowing. During the preparation of gellan gum, particularly in the preparation of high acyl gellan gum, in order to remove the yellow pigments from the colloid, it needs to increase the amount of ethanol or isopropanol used in decoloration by extraction (in general, ethanol or isopropanol is used at the amount based on volume of 3 times of the volume of the fermentation broth), and the time for the operation. This reduces the efficiency of extraction and purification, leading to an increase on costs